1. Field of the Invention
The present invention relates to a method for forming a plug structure, and more particularly to a method of cleaning an opening by using a punching through process.
2. Description of the Prior Art
Currently, semiconductor devices are widely involved in many products and services in our daily life. These semiconductor devices are fabricated through many processes, such as photolithography, deposition, ion implantation, or etching, to form a plurality of integrated circuit (IC) devices on a wafer. In semiconductor fabrication on a wafer, an opening with a high aspect ratio, which is defined as a ratio of the depth to the width, is needed in some situations. The opening, such as a via hole or a contact hole, is formed in a dielectric layer and is filled with a metallic material, such as tungsten, to form a plug.
Please refer to FIGS. 1-3. FIGS. 1-3 are schematic cross-sectional diagrams illustrating a method for forming a contact plug in a wafer 10 according to the prior art. As shown in FIG. 1, a wafer 10 is provided first. The wafer 10 includes a substrate 12, a plurality of metal-oxide-semiconductor (MOS) devices 14 formed on the substrate 12, a dielectric layer 16 covering the substrate 12, and a plurality of plug holes 18 formed in the dielectric layer 16. There are usually some unwanted oxides 50, such as native oxides, formed on the surface of the MOS devices 14. These oxides 50 will interfere the electrical connection between the MOS devices 14 and the subsequently formed contact plug.
As shown in FIG. 2, in order to remove the oxides 50, an argon (Ar) cleaning process is subsequently carried out before a glue layer is formed. As the industry progresses into submicron processing techniques, a higher aspect ratio is needed for the contact plug, and it is therefore harder to completely remove all the oxides 50. As a result, the argon cleaning process should be enhanced, or the process time of the argon cleaning process should be extended to clean the bottom oxides 50. However, when the argon cleaning process is enhanced, or the process time of the argon cleaning process is extended, a portion of the dielectric layer 16 positioned around the top corner of the plug hole 18 might be over-rounded. More the argon cleaning process is performed, more seriously the top corner of the plug hole 18 is over-rounded.
As shown in FIG. 3, a titanium layer 22 is afterward formed over the dielectric layer 16 by a physical vapor deposition (PVD) process, a titanium nitride layer 24 is next formed over the titanium layer 22 by a chemical vapor deposition (CVD) process, and a tungsten layer 26 is formed over the wafer 10 to fill the plug hole 18. Thereafter, a chemical mechanical polishing (CMP) process is performed to remove excessive parts of the tungsten layer 26, excessive parts of the titanium nitride layer 24, and excessive parts of the titanium layer 22, so as to form contact plugs 28 in the dielectric layer 16. The titanium layer 22 or the titanium nitride layer 24 functions as a glue layer or a barrier layer around the contact plugs 28.
Because the top corner of the plug hole 18 is over-rounded, a barrier bridge problem 30 is caused between contact plugs 28, as shown in FIG. 3, especially when the critical dimension (CD) becomes smaller. The titanium layer 22 or the titanium nitride layer 24 electrically connects two independent contact plugs 28 with each other, and causes a short circuit in the wafer 10. Therefore, the fabricated products fail in performance.
In order to prevent the barrier bridge problem 30 and remove the oxides 50, an F-base cleaning process is carried out instead of the argon cleaning process, as shown in FIG. 4. However, the F-base selectivity with SiO2 and SiN is poor, so it is easy to cause the CD of the contact plug 28 to be enlarged, especially at the SiN spacer or a SiN contact etch stop layer (CESL). It is easy to widen the contact plug 28 to reach the poly-gate of the MOS device 14, and causes a short circuit between the contact plug 28 and the poly-gate.
The existence of oxides reduces the device performance, and the short circuit even causes a failure of the device. Accordingly, it is desired to provide a cleaning method that does not deform the opening.